Diabetes mellitus can be divided into two clinical syndromes, Type I and Type II diabetes mellitus. Type I diabetes, or insulin-dependent diabetes mellitus, is a chronic autoimmune disease characterized by the extensive loss of beta cells in the pancreatic islets of Langerhans (hereinafter referred to as “pancreatic islet cells” or “islet cells”), which produce insulin. As these cells are progressively destroyed, the amount of secreted insulin decreases, eventually leading to hyperglycemia (abnormally high level of glucose in the blood) when the amount secreted drops below the level required for euglycemia (normal blood glucose level). Although the exact trigger for this immune response is not known, patients with Type I diabetes have high levels of antibodies against pancreatic beta cells (hereinafter “beta cells”). However, not all patients with high levels of these antibodies develop Type I diabetes.
Type II diabetes, or non-insulin-dependent diabetes mellitus, develops when muscle, fat and liver cells fail to respond normally to insulin. This failure to respond (called insulin resistance) may be due to reduced numbers of insulin receptors on these cells, or a dysfunction of signaling pathways within the cells, or both. The beta cells initially compensate for this insulin resistance by increasing their insulin output. Over time, these cells become unable to produce enough insulin to maintain normal glucose levels, indicating progression to Type II diabetes (Kahn S E, Am J Med (2000) 108 Suppl 6a, 2S-8S).
The fasting hyperglycemia that characterizes Type II diabetes occurs as a consequence of the combined lesions of insulin resistance and beta cell dysfunction. The beta cell defect has two components: the first component, an elevation of basal insulin release (occurring in the presence of low, non-stimulatory glucose concentrations), is observed in obese, insulin-resistant pre-diabetic stages as well as in Type II diabetes. The second component is a failure to increase insulin release above the already elevated basal output in response to a hyperglycemic challenge. This lesion is absent in prediabetes and appears to define the transition from normo-glycemic insulin-resistant states to frank diabetes. There is currently no cure for diabetes. Conventional treatments for diabetes are very limited, and focus on attempting to control blood glucose levels in order to minimize or delay complications. Current treatments target either insulin resistance (metformin, thiazolidinediones (“TZDs”)), or insulin release from the beta cell (sulphonylureas, exenatide). Sulphonylureas, and other compounds that act by depolarizing the beta cell, have the side effect of hypoglycemia since they cause insulin secretion independent of circulating glucose levels. One approve drug, Byetta (exenatide) stimulates insulin secretion only in the presence of high glucose, but is not orally available and must be injected. Januvia (sitagliptin) is another recently approved drug that increases blood levels of incretin hormones, which can increase insulin secretion, reduce glucagon secretion and have other less well characterized effects. However, Januvia and other dipeptidyl peptidases IV inhibitors may also influence the tissue levels of other hormones and peptides, and the long-term consequences of this broader effect have not been fully investigated. There is an unmet need for oral drugs that stimulate insulin secretion in a glucose dependent manner.
Progressive insulin resistance and loss of insulin secreting pancreatic beta cells are primary characteristics of Type II diabetes. Normally, a decline in the insulin sensitivity of muscle and fat is compensated for by increases in insulin secretion from the beta cell. However, loss of beta cell function and mass results in insulin insufficiency and diabetes (Kahn B B, Cell 92:593-596, 1998; Cavaghan M K, et al., J Clin Invest 106:329-333, 2000; Saltiel A R, Cell 104:517-529, 2001; Prentki M and Nolan C J, J Clin Invest 116:1802-1812 (2006); and Kahn S E, J Clin Endocrinol Metab 86:4047-4058, 2001). Hyperglycemia further accelerates the decline in beta cell function (UKPDS Group, JAMA 281:2005-2012, 1999; Levy J, et al., Diabetes Med 15:290-296, 1998; and Zhou Y P, et al., J Biol Chem 278:51316-23, 2003). Several of the genes in which allelic variation is associated with an increased risk of Type II diabetes are expressed selectively in the beta cell (Bell G I and Polonsky K S, Nature 414:788-791 (2001); Saxena R, et al., Science (2007) Apr. 26; and Valgerdur Steinthorsdottir, et al., Nature Genetics (2007) Apr. 26).
Insulin secretion from the beta cells of pancreatic islets is elicited by increased levels of blood glucose. Glucose is taken up into the beta cell primarily by the beta cell and liver selective transporter GLUT2 (Thorens B, Mol Membr Biol 2001 October-December; 18(4):265-73). Once inside the cell, glucose is phosphorylated by glucokinase, which is the primary glucose sensor in the beta cell since it catalyzes the irreversible rate limiting step for glucose metabolism (Matschinsky F M, Curr Diab Rep 2005 June; 5(3):171-6). The rate of glucose-6-phosphate production by glucokinase is dependent on the concentration of glucose around the beta cell, and therefore this enzyme allows for a direct relationship between level of glucose in the blood and the overall rate of glucose oxidation by the cell. Mutations in glucokinase produce abnormalities in glucose dependent insulin secretion in humans giving further evidence that this hexokinase family member plays a key role in the islet response to glucose (Gloyn A L, et al., J Biol Chem 2005 Apr. 8; 280(14):14105-13, Epub 2005 Jan. 25). Small molecule activators of glucokinase enhance insulin secretion and may provide a route for therapeutic exploitation of the role of this enzyme (Guertin K R and Grimsby J, Curr Med Chem 2006; 13(15):1839-43; and Matschinsky F M, et al., Diabetes 2006 January; 55(1):1-12) in diabetes. Glucose metabolism via glycolysis and mitochondrial oxidative phosphorylation ultimately results in ATP production, and the amount of ATP produced in a beta cell is directly related to the concentration of glucose to which the beta cell is exposed.
Glucose dependent insulin secretion from the beta cell is dependent on numerous neurotransmitters and blood-borne hormones, as well as local, intra-islet factors. CNS activation of the vagal innervation of the islet can lead to the release of small molecules such as acetylcholine and peptides such as vasoactive intestinal polypeptide (VIP), gastrin releasing peptide (GRP) and Pituitary Adenylate Cyclase Activating Peptide (PACAP). Acetylcholine activation of phospholipase C through the Gαq-coupled GPCR M3 muscarinic receptor leads to release of Ca2+ from intracellular stores (Gilon P and Henquin J C, Endocr Rev 2001 October; 22(5):565-604). Cholinergic agonists also lead to a subtle Na+-dependent plasma membrane depolarization that can work in concert with glucose-initiated depolarization to enhance insulin release (Gilon P and Henquin J C, Endocr Rev 2001 October; 22(5):565-604). VIP and PACAP each bind to an overlapping set of Gα-coupled GPCRs (PAC1, VIPR1, and VIPR2) on the beta cell that lead to stimulation of adenylate cyclase and an increase in intracellular cAMP (Filipsson K, et al., Diabetes 2001 September; 50(9):1959-69; Yamada H, et al., Regul Pept 2004 Dec. 15; 123(1-3):147-53; and Qader S S, et al., Am J Physiol Endocrinol Metab 2007 May; 292(5):E1447-55).
Incretin hormones such as Glucagon-Like Peptide 1 (GLP-1) and Glucose-dependent Insulinotropic Polypeptide (GIP, also known as Gastric Inhibitory Polypeptide) also bind to specific Galphas-coupled GPCRs receptors on the surface of islet cells, including beta cells, and raise intracellular cAMP (Drucker D J, J Clin Invest 2007 January; 117(1):24-32). Although the receptors for these hormones are present in other cells and tissues, the overall sum of effects of these peptides appear to be beneficial to control of glucose metabolism in the organism (Hansotia T, et al., J Clin Invest 2007 January; 117(1):143-52, Epub 2006 Dec. 21). GIP and GLP-1 are produced and secreted from intestinal K and L cells, respectively, and these peptide hormones are released in response to meals by both direct action of nutrients in the gut lumen and neural stimulation resulting from food ingestion. GIP and GLP-1 have short half-lives in human circulation due to the action of the protease dipeptidyl-peptidase IV (DPPIV), and inhibitors of this protease can lower blood glucose due to their ability to raise the levels of active forms of the incretin peptides. The glucose lowering that can be obtained with DPPIV inhibitors, however, is somewhat limited since these drugs are dependent on the endogenous release of the incretin hormones. Peptides (e.g., exanatide (Byetta)) and peptide-conjugates that bind to the GIP or GLP-1 receptors but are resistant to serum protease cleavage can also lower blood glucose substantially (Gonzalez C, et al., Expert Opin Investig Drugs 2006 August; 15(8):887-95), but these incretin mimetics must be injected and tend to induce a high rate of nausea and therefore are not ideal therapies for general use in the Type II diabetic population. The clinical success of DPPIV inhibitors and incretin mimetics, though far from ideal, do point to the potential utility of compounds that increase incretin activity in the blood. Some studies have indicated that beta cell responsiveness to GIP is diminished in Type II diabetes (Nauck M A, et al., J Clin Invest 91:301-307 (1993); and Elahi D, et al., Regul Pept 51:63-74 (1994)). Restoration of this responsiveness (Meneilly G S, et al., Diabetes Care 1993 January; 16(1):110-4) may be a promising way to improve beta cell function in vivo.
Since increased incretin activity has a positive effect on glucose dependent insulin secretion and perhaps other mechanisms that lead to lower blood glucose, it is also of interest to explore therapeutic approaches to increasing incretin release from intestinal K and L cells. GLP-1 secretion appears to be attenuated in Type II diabetes (Vilsboll T, et al., Diabetes 50:609-613), so improving incretin release may ameliorate this component of metabolic dysregulation. Nutrients such as glucose and fat in the gut lumen prompt incretin secretion by interaction with apical receptors (Vilsboll T, et al., Diabetes 50:609-613). GLP-1 and GIP release can also result from neural stimulation; acetylcholine and GRP can enhance incretin release in a manner perhaps analogous to the effects of these neurotransmitters on the beta cell in regard to insulin secretion (Brubaker P, Ann NY Acad Sci 2006 July; 1070:10-26; and Reimann F, et al., Diabetes 2006 December; 55(Suppl 2):S78-S85). Somatostatin, leptin and free fatty acids also appear to modulate incretin secretion (Brubaker P, Ann NY Acad Sci 2006 July; 1070:10-26; and Reimann F, et al., Diabetes 2006 December; 55(Suppl 2):S78-S85). To date, however, there does not appear to be a way to selectively impact these pathways to promote incretin secretion for therapeutic benefit. There is a need for oral drugs that stimulate incretin secretion in the treatment of diabetes.
Incretins can also increase the rate of beta cell proliferation and decrease the apoptotic rates of beta cells in animal models (Farilla L, et al., Endocrinology 2002 November; 143(11):4397-408) and human islets in vitro (Farilla L, et al., Endocrinology 2003 December; 144(12):5149-58). The net result of these changes is an increase in beta cell number and islet mass, and this should provide for increased insulin secretory capacity, which is another desired aim of anti-diabetic therapies. GLP-1 has also been shown to protect islets from the destructive effects of agents such as streptozotocin by blocking apoptosis (Li Y, et al., J Biol Chem 2003 Jan. 3; 278(1):471-8). Cyclin D1, a key regulator of progression through the cell cycle, is up-regulated by GLP-1, and other agents that increase cAMP and PKA activity also have a similar effect (Friedrichsen B N, et al., J Endocrinol 2006 March; 188(3):481-92; and Kim M J, et al., J Endocrinol 2006 March; 188(3):623-33). Increased transcription of the cyclin D1 gene occurs in response to PKA phosphorylation of CREB (cAMP-response element binding) transcription factors (Hussain M A, et al., Mol Cell Biol 2006 October; 26(20):7747-59). There is a need for oral drugs that increase beta cell number and islet mass in the treatment of diabetes.
G protein-coupled receptors (GPCRs) are cell-surface receptors that play an important physiological role by transducing and amplifying extra-cellular signals such as hormones, growth factors, neurotransmitters and physiologically active substances. GPCRs are associated with changes in intracellular Ca2+ concentration as well as increases in intracellular inositol 1,4,5-triphosphate (IP3) concentration. These second messengers serve to focus the signal transduction events and stimulate other pathways. Hence, GPCRs are therapeutically important target classes in the pharmaceutical industry.
GPR120 is a GPCR for unsaturated long-chain free fatty acids (FFA) and is highly expressed in lung, intestine, adipocytes and taste cells as well as in the enteroendocrine cell lines such as STC-1 and GLUTag (Hirasawa et al., Nature Medicine 2005 January; 11: 90-94; and Iakoubov et al., Endocrinology 2007 March; 148(3): 1089-1098; and Katsuma et al., J. Biol. Chem. 2005 May; 280:19507-19515; Matsumura et al., Biomed. Res. 2007 February; 28(1) 49-55). The stimulation of GPR120 by FFAs increases the release of Ca2+ from intracellular stores indicating that GPR120 is a Gαq-coupled receptor. GPR120 mediates the effect of unsaturated long-chain free fatty acids in stimulating GLP-1 and cholecystokinin (CCK) secretion, increases plasma insulin, activation of the extracellular signal-regulated kinase (ERK) cascade, proliferation of pancreatic β cells, inhibition of serum deprivation-induced apoptosis and adipogenesis (Katsuma et al., J. Biol. Chem. 2005 May; 280:19507-19515; and Rayasam et al., Expert Opin. Ther. Targets 2007 May; 11(5): 661-671; and Tanaka et al., Naunyn Schmiedeberg Arch Pharmacol 2008 June; 377(4-6):515-522; and Gotoh et al., Biochem. Biophys. Res. Commun. 2007 March; 354(2): 591-597).
Free fatty acids have been demonstrated as ligands for recently identified orphan GPCRs (Rayasam et al., Expert Opin Ther Targets 2007 May; 11(5):661-671). GPR120 shares ligand specificity with other fatty acid receptors and there is a need for the development of small molecule agents that are specific modulators for GPR120 function. In particular, GPR120 is a promising target for the treatment of diabetes, obesity and the metabolic syndrome considering the significant role of GLP-1 and CCK in insulin secretion, gastric emptying and appetite feeding control.